Valve drive adjustment devices controlled by an electronic engine control unit in a motor vehicle are known in a variety of applications. Three are mentioned here byway of example. In the case of the “Valvetronic” adjustment device (BMW), an actuating mechanism arranged between a camshaft and a valve tappet is adjusted by means of an electric actuating motor and an eccentric shaft in order to realize a variable (maximum) valve lift as a function of certain operating parameters of the internal combustion engine. Said adjustment of the valve lift may be combined with a so-called camshaft phase adjustment in which a relative adjustment of the crankshaft and camshaft with respect to one another, and thus a variation of the valve timing, is also performed as a function of the operating state of the internal combustion engine. An electronically controlled change of the valve actuation characteristic by means of an engine control unit is likewise provided in the case of the “VTEC” (Honda) adjustment device. Said system exists in different variants which however have in common the fact that a switch between different actuation characteristics takes place as a function of certain parameters of engine operation. In the “Ti-VCT” (Ford) adjustment device, adjustment by means of hydraulic actuators is provided. The adjustment is performed in a continuously variable fashion as a function of engine load by means of electronic characteristic-map-based control.
The known valve drive adjustment devices may furthermore be divided roughly into two categories with regard to variability of adjustment, specifically discontinuously variable systems and continuously variable systems.
A discontinuously variable system may for example be constructed such that a plurality of different profiles per valve is provided on the camshaft. Each profile results in a different lift curve for the valve, wherein at all times only one profile can be active, and switching between the different profiles can be performed. In concepts of said type, a change in the lift curve is performed abruptly.
In relation thereto, continuously variable systems offer more comprehensive variability of the valve drive. Since said systems permit a continuous variation of the valve actuation characteristic, they thus eliminate for example the difficulties that otherwise result from an abrupt switch. Continuous systems are used in particular for low-loss load control of internal combustion engines. It is for example possible for the air or mixture supply to the combustion chamber to be controlled by means of the variability of the lift curve of an inlet valve.
In particular in the case of continuously variable systems which are controlled by means of an electronic control unit, the most precise possible knowledge of the present valve lift or of the present setting of the system is necessary in order to permit correct and fast control or regulation of the system and thus optimally control the combustion. In the case of known valve drive adjustment devices, therefore, a position sensor (for example rotary angle transducer etc.) is provided which outputs to the control unit a position sensor signal which represents the present position (setting) of the adjustment device.
Such a position sensor signal is then taken as an input variable in the control, in particular for example regulation, of the valve drive adjustment device by the electronic engine control unit.
The arrangement of a position sensor on the valve drive adjustment device however disadvantageously entails a certain amount of outlay in terms of construction, in particular if it is sought for the sensor signal output by said position sensor to represent the actual present setting with high accuracy.